Seismic exploration can provide valuable information useful in the drilling and operation of oil and gas wells. In seismic exploration, energy is introduced by a seismic source on the earth's surface to create a seismic signal that is propagated throughout the subterranean formation. This seismic signal is reflected to differing degrees by features that are of interest. A receiver on the surface monitors these reflected signals to help generate a seismic map of the underground features. This map is generated by knowing the exact time that a seismic signal was activated as compared to the time that the reflected signal is received. As a practical matter, the system comprises a plurality of sources and receivers to provide the most comprehensive map possible of subterranean features. Different configurations may yield two dimensional or three dimensional results depending on their mode of operation.
While these calculations may seem simple at first glance, difficulty arises from the fact that the seismic signals will travel at different velocities depending on the makeup of the formation. This leads to an image of the reflecting feature that is unfocused and provides a depth that is likely to be inaccurate.
In order to correct for this inaccuracy, it is necessary to create a velocity profile of the formation surrounding a borehole so that the expected velocities can be used in determining the depth and size of the underground feature of interest. This can be achieved using a “checkshot” which typically involves a seismic source on or near the surface and a receiver placed at a known distance in the already drilled borehole, although “reverse checkshots” are also known in the art. The source then generates a seismic signal and the receiver records the time at which it reaches that point in the borehole. Since all distances are known, the velocity of the signal's travel through the borehole can be calculated.
This checkshot and velocity calculation must be performed at a plurality of depths in the borehole and using one or more sources at the same location or spaced at varying distances from the borehole. Conventional systems require that the receiver in the borehole be stopped and anchored for each checkshot measurement. This greatly increases the time and cost required in creating a velocity profile. The reason for this is the fact that the electronics and sensors used in receiving the seismic signals are too sensitive to any noise from contact, such as abrasion, with the borehole wall, and from the motion of the receiver. Additionally, due to the same considerations, the receiver generally has to be introduced into the borehole on its own with only its associated electronics.